Ultrasonic waves can be used for inspecting a structure, such as a pipe, to identify defects and flaws within the structure. Examples of ultrasonic inspection devices can be found in WO 96/12951 A, WO 2007/125308 A2 and EP 1 394 538 A1
WO 96/12951 A describes apparatus for inspecting elongate members, such as pipes. The apparatus includes a ring of angularly-spaced transducers (or “exciters”) clamped to the outside wall of a pipe. Each transducer includes a piezoelectric element, a metal block adhesively bonded to the piezoelectric element, and a thin faceplate shim secured to the face of the piezoelectric element to provide a wear plate.
In the prior art transducer, the piezoelectric element and wear plate are stacked in a recess in the metal block and secured using an epoxy adhesive. This arrangement can have disadvantages.
Firstly, the sides of the wear plate may be constrained by the sides of the recess. This can decrease the efficiency of the transducer.
Secondly, the metal block is near the pipe. If the pipe is hot, then the metal block can absorb heat radiated by the pipe and heat the piezoelectric element and the epoxy adhesive. Piezoelectric elements have a maximum operating temperature and so this arrangement can limit the maximum temperature of pipe that can be inspected using the transducer.
Thirdly, when inspecting a hot pipe, both the front of the metal block and the wear plate are exposed to high temperatures. If the metal block and wear plate are made from different materials, then they may expand at different rates dues to different coefficients of thermal expansion. This can limit the maximum operating temperature of the transducer and reduce the number of operation cycles.
Finally, the bond line between the metal block and the wear plate is at the front of the transducer and so is usually exposed to the environment. This can limit the lifetime of the transducer and/or make it less reliable due to exposure to the environment, for example due to ingress of sea water. Furthermore, if the pipe is hot, then the epoxy adhesive is directly exposed to heating.